Solution mining a stable roof under an inert gas

ABSTRACT

This method includes a solution mined underground salt cavern, wherein the salt cavern has a main body with a mean diameter of D N , and an upper portion comprising an inert gas pad, a stream of leaching water which is injected below the inert gas pad with a velocity V, thereby leaching an Nth tier adjacent to the upper portion having a height H 1  and a mean diameter D N+1  that is smaller than D N  by a ratio R raising the inert gas pad by an amount A 1 , providing a stream of leaching water which is injected below the inert gas pad with a velocity V, thereby leaching a N+1th tier adjacent to the Nth tier having a height H 2  and a to a mean diameter D N+2  that is smaller than D N+1  by a ratio R, and repeating steps c and d a number of times T, thereby forming a stable roof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 (a) and (b) to U.S. Provisional Patent Application No. 62/089,564 filed Dec. 9, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

Leached storage caverns in salt formations typically have a relatively flat roof. Large diameter flat roofs in a salt cavern can be unstable due to the low tensile strength of the salt, salt movement, fractured salt or low pressure in the cavern. The stability of the roof may be increased by leaching a modified dome shape in the roof.

The leaching of storage caverns in salt formations is typically performed under a blanket of liquid hydrocarbons. Some storage applications may require very clean or ultra pure caverns, where residual hydrocarbons could contaminate the stored product. To prevent these contamination issues, ultra pure salt caverns can be leached under an inert gas blanket. This invention claims that to increase the overall stability of the roof of a leached salt cavern, a domed roof is leached using in an inert gas blanket

SUMMARY

This method includes a solution mined underground salt cavern, wherein the salt cavern has a main body with a mean diameter of D_(N), and an upper portion comprising an inert gas pad, a stream of leaching water which is injected below the inert gas pad with a velocity V, thereby leaching an Nth tier adjacent to the upper portion having a height H1 and a mean diameter D_(N+1) that is smaller than D_(N) by a ratio R raising the inert gas pad by an amount A1, providing a stream of leaching water which is injected below the inert gas pad with a velocity V, thereby leaching a N+1th tier adjacent to the Nth tier having a height H2 and a to a mean diameter D_(N+2) that is smaller than D_(N+1) by a ratio R, and repeating steps c and d a number of times T, thereby forming a stable roof.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 illustrates an embodiment of the invention;

FIG. 2 illustrates an embodiment of the invention;

FIG. 3 illustrates an embodiment of the invention;

FIG. 4 illustrates an embodiment of the invention;

FIG. 5 illustrates an embodiment of the invention; and

FIG. 6 illustrates an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

This invention claims that to increase the overall stability of the roof of a leached salt cavern, a domed roof 204 is leached using in an inert gas blanket 102, which could be nitrogen, helium, argon or methane. The inert gas blanket is injected into the outer annulus string 106 of a salt cavern 101. The blanket pressure is maintained at a pressure above the water injection pressure but less than the maximum pressure for the cavern as defined by the depth of the final cemented casing shoe and the maximum pressure gradient for the cavern.

The blanket depth may be controlled by monitoring the blanket gas pressure and by verification of the blanket depth may be by wire line density measurement for the gas-brine interface. Inert gas depth may be raised in increments of between 20 to 40 ft at a time. The cavern roof may be leached to a diameter 20 to 30 percent less than the old essentially flat roof. Once the new roof is leached to the smaller diameter, the inert gas blanket is raised another 20 to 40 ft and the cavern roof is leached to a diameter 20 to 30 percent less than the old roof. This process continues until the final geometry of the cavern approximates a dome. Geometry of the roof is verified by through pipe sonar.

It is further claimed that the geometry of the storage cavern roof may be controlled by the flow of water 103 into the cavern. The water injection flow into the cavern may be maintained between the minimum flow rate of 5 ft/sec velocity and the maximum flow rate of 8 ft/sec. Ideal cavern roof geometry is achieved by flowing at a constant flow rate of approximately between 6 and 7 ft/sec.

Turning to FIG. 1, a solution mined underground salt cavern 101 has a main body with a mean diameter of D₁, and an upper portion comprising an inert gas pad 102. The inert gas is selected from the group consisting of nitrogen, helium, argon, or methane. A stream of leaching water 103 is injected below inert gas pad 102 with a velocity V. Velocity V may be between 5 feet/second and 9 feet per second, preferably V may be between 6 feet/second and 7 feet/second.

The inert gas pad has a pressure, and a depth defined by the interface between the inert gas and a brine/water mixture produced by the solution mining, and the inert gas pad depth may be determined by monitoring the gas pad pressure. The inert gas pad depth may be verified by wire line density measurement at the interface.

Now turning to FIG. 2, inert gas pad height is raised H1 feet. H1 may be between 20 feet and 40 feet, preferably between 25 feet and 35 feet, more preferably 30 feet. As leaching water 103 is injected, it now reaches region 104, which had previously been protected from leaching by inert gas pad 102, thereby solution mining this portion of the roof of cavern 101.

Turning to FIG. 3, leaching water 103 produces a first tier 201 in the top of cavern 101. First tier 201 has a height H1 above the nominal roof of the cavern R, and a mean diameter D₂ that is smaller than D₁ by a predetermined ratio R. The ratio R bay be between 15% and 35%, preferably it may be between 20% and 30%, even more preferably it may be 25%.

Now turning to FIG. 4, inert gas pad height is raised H2 feet. H2 may be between 10 feet and 50 feet, preferably between 20 feet and 40 feet, preferably between 25 feet and 35 feet, more preferably 30 feet. As leaching water 103 is injected, it now reaches region 105, which had previously been protected from leaching by inert gas pad 102, thereby solution mining this portion of the roof of cavern 101.

Turning to FIG. 5, leaching water 103 produces a second tier 202 in the top of cavern 101. First tier 202 has a height H2 above first tier 201, and a mean diameter D₃ that is smaller than D₂ by a predetermined ratio R. The ratio R bay be between 15% and 35%, preferably it may be between 20% and 30%, even more preferably it may be 25%.

As indicated in FIG. 6, a stream of leaching water which is injected below the inert gas pad with a velocity V leaches a N+1th tier adjacent to the Nth tier having a height H2 and a to a mean diameter D_(N+2) that is smaller than D_(N+1) by a predetermined ratio R. These steps are repeated a predetermined number of times T, thereby forming a stable, dome shaped roof. 

What is claimed is:
 1. A method of solution mining a stable roof under an inert gas blanket, comprising: a) providing a solution mined underground salt cavern, wherein said salt cavern has a main body with a mean diameter of D_(N), and an upper portion comprising an inert gas pad, b) providing a stream of leaching water which is injected below the inert gas pad with a velocity V, thereby leaching an Nth tier adjacent to the upper portion having a height H1 and a mean diameter D_(N+1) that is smaller than D_(N) by a predetermined ratio R, c) raising the inert gas pad by a predetermined amount A1, d) providing a stream of leaching water which is injected below the inert gas pad with a velocity V, thereby leaching a N+1th tier adjacent to the Nth tier having a height H2 and a to a mean diameter D_(N+2) that is smaller than D_(N+1) by a predetermined ratio R, e) repeating steps c and d a predetermined number of times T, thereby forming a stable roof.
 2. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein the inert gas is selected from the group consisting of nitrogen, helium, argon, or methane.
 3. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein the inert gas is nitrogen.
 4. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein H2 is between 10 and 50 feet.
 5. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein H2 is between 20 and 40 feet.
 6. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein H2 is 30 feet.
 7. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein R is between 15% and 35%.
 8. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein R is between 20% and 30%.
 9. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein R is 25%.
 10. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein V is between 5 feet/second and 9 feet per second.
 11. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein V is between 6 feet/second and 7 feet/second.
 12. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein the inert gas pad has a pressure, and a depth defined by the interface between the inert gas and a brine/water mixture produced by the solution mining, and wherein the inert gas pad depth is determined by monitoring the gas pad pressure.
 13. The method of solution mining a stable roof under an inert gas blanket of claim 1, wherein the inert gas pad has a depth defined by the interface between the inert gas and a brine/water mixture produced by the solution mining, and wherein the inert gas pad depth is verified by wire line density measurement at the interface. 